(1) Field of the Invention
The present invention relates to a vehicle headlamp having a vent hole.
(2) Description of the Prior Art
A prior art vehicle headlamp having a vent hole is basically constructed such as shown in FIG. 6. A drying agent 22 such as silica gel is filled in a container 21 having small holes 21a and 21b in opposite ends thereof, the container 21 extending through the housing (reflector) 23. Reference numeral 24 designates a lens of the prior art headlamp.
In the vehicle headlamp provided with the vent hole constructed as described above, a pressure rise in the headlamp is suppressed by the vent hole. Further, air coming from the outside is dehumidified by the drying agent 22.
The drying agent 22, which is in the state of moisture absorption by the dehumidification of air coming from the outside, is dried by heat generated when the lamp is lit, to thereby release water content thereof. That is, a recycle effect is provided.
To minimize the contact with open air, a porous resin film 25 having a permeability is disposed on the end of the housing 23 which is external of the container 21 as shown in FIG. 7, or means such as a mechanical valve 26 as shown in FIG. 8 is provided on the external end of container 21.
However, where the recycle effect is provided for the drying agent 22 as described above, it can be expected that the water content released from the drying agent 22 due to the temperature rise when the lamp is lit is discharged to the outside of the lamp due to the internal pressure of the headlamp. But, since a flow of air flowing out of the headlamp stops after a lapse of a fixed time, if the moisture absorption of the drying agent 22 is large, the release of water content continuously takes place thereafter, and the water content also flows into the headlamp through the small hole 21a in the end of the container 21 which is inside the housing 23.
At this time, the interior of the container 21 is in the state of high temperature and high humidity, and the air of the high temperature and high humidity flows into the housing 23 without the air of the high temperature and high humidity being humidity-absorbed into the drying agent 22, by the air flowing therein as the lamp is deenergized.
On the other hand, where the porous resin film 25 (FIG. 7) or the valve 26 (FIG. 8) is provided in order to cut off the open air, the discharge of the releasing water content to the outside of the lamp during the above-mentioned venting course is not only impaired by the film 25 or the valve 26 but also conversely the flow of water content into the inside of the lamp is accelerated. Moreover, the mechanical valve construction (of FIG. 8) must function to control the flow of air passing through the vent hole, as a consequence of which the construction becomes complicated and increased in size, leading to higher cost.